1. Field of the Invention
The present invention relates to an autofocusing zoom lens which includes a distance measurement unit provided separately from a photographic optical system and configured to execute focusing.
2. Description of the Related Art
In recent years, a broadcast high-definition (HD) recording system has been widely used and a large-size home monitor has been used. Under these circumstances, sufficient focusing accuracy cannot be achieved by merely executing manual focusing as in a conventional method. Accordingly, a problem of a defocused image often occurs. To solve the above-described problem, it is desired that a lens for an HD system have a highly accurate autofocusing function.
As an autofocusing method, a contrast autofocusing method, which uses a video signal from an imaging unit, such as a charge-coupled device (CCD) sensor, is widely known. In addition, an external light autofocusing method, which uses a distance measurement unit that executes a trigonometric distance measurement operation, is also widely known.
For example, Japanese Patent No. 3738795 discusses a method that uses the contrast autofocusing method and the external autofocusing method described above in combination. More specifically, the conventional method switches the autofocusing methods according to various conditions, such as the temperature inside or outside the apparatus, an aperture value, or a focal length.
The zoom lens discussed in Japanese Patent No. 3738795 is a rear-focus type zoom lens capable of executing focusing with a high accuracy by the contrast autofocusing method after roughly executing focusing by the external autofocusing method. With this configuration, the zoom lens discussed in Japanese Patent No. 3738795 can achieve quick and highly accurate focusing.
However, in the broadcasting industry, in which a user is particular about shooting a normal image, the user is interested in the normality of the image rather than the speed of focusing.
In the contrast autofocusing method, an operation for focusing is necessary in calculating a maximum value of contrast. Therefore, an abnormality of a video, such as overshooting or vibrated reproduction of a defocused image, may occur. Accordingly, it is desired that the contrast autofocusing method be used under restricted conditions of use.